April 10, 2003
09:00 AM (EDT)

News Release Number: STScI-2003-12

Far-Flung Supernovae Shed Light on Dark Universe

April 10, 2003: New Hubble Space Telescope observations of a pair of very distant
exploding stars, called Type Ia supernovae, provide new clues about the
accelerating universe and its mysterious "dark energy." Astronomers used
the telescope's Advanced Camera for Surveys to help pinpoint the
supernovae, which are approximately 5 billion and 8 billion light-years
from Earth. The farther one exploded so long ago the universe may still
have been decelerating under its own gravity.

Q & A: Understanding the Discovery

1.
How did the telescope's Advanced Camera for Surveys help astronomers find the exploding stars? And why do astronomers use these exploding stars to probe the early universe?

The camera's powerful vision picked out the faint glow of the distant
supernovae. Then it dissected the light from the exploding stars to
measure the distances and to study how they faded over time. After
careful analysis, astronomers confirmed that these supernovae are a
special kind of exploding star, Type Ia supernovae. Astronomers call
them reliable distance indicators because when they explode, they glow
at a predictable peak brightness. Knowing how bright they actually are
and how bright they appear to be through a telescope, astronomers are
able to calculate their distance from Earth.

Examining the light from distant exploding stars can shed light on the
early universe. Information from studies of Type Ia supernovae about
five years ago revealed the stunning news that galaxies appeared to be
moving away from each other at an ever-increasing speed. They have
attributed this accelerating expansion to a mysterious factor, a dark
energy believed to permeate the universe.

But astronomers also found evidence that dark energy wasn't always in
control. In 2001 studies of a far-flung supernova yielded the first
evidence that the expanding universe was once decelerating, that gravity
was slowing the universe's expansion. They have very little data,
though, on the period of transition between these two phases, when the
repulsion produced by dark energy began to surpass the tug of gravity.